Ooh, that’s a big one,” Donald Trump said Monday as he signed an executive order – one of dozens during his first hours as president – to withdraw the United States from the World Health Organization.

The U.S. has traditionally been the most generous benefactor of the WHO. A Trump executive order to cut ties with the WHO could pose a threat to global public health.

Public health experts say U.S. withdrawal from the W.H.O. would undermine the nation’s standing as a global health leader and make it harder to fight the next pandemic.